Compares four different doses of melatonin and placebo in elderly and young subjects. Melatonin is a hormone produced by the pineal gland in the brain at night. Its secretion closely reflects the timing of the circadian pacemaker in the brain. By measuring this we are able to detect what "time it is" in the brain. Four different doses of melatonin are given to two age groups to see how quickly melatonin is removed from the body and if there are any differences between these two groups.